There is a system include file TargetConditionals.h on macOS, which
defines TARGET_OS_LINUX (and others) to 0 or 1. On the other side,
TARGET_OS_LINUX is also defined by trivia/config.h.cmake, but there it
has another possible values: undefined or 1. This inconsistency causes
issues like #8445, when TARGET_OS_LINUX is defined (to 0) in one file
and undefined in another. Let's always define it to 0 or 1.

Closes #8445

NO_DOC=bugfix

(cherry picked from commit 19b53ac3)

getenv() return values cannot be trusted, because an attacker might set
them. For instance, we shouldn't expect, that getenv() returns a value
of some sane size.

Another problem is that getenv() returns a pointer to one of
`char **environ` members, which might change upon next setenv().

Introduce a wrapper, getenv_safe(), which returns the value only when
it fits in a buffer of a specified size, and copies the value onto the
buffer. Use this wrapper everywhere in our code.

Below's a slightly decorated output of `grep -rwn getenv ./src --include
*.c --include *.h --include *.cc --include *.cpp --include *.hpp
--exclude *.lua.c` as of 2022-10-14.
`-` marks invalid occurences (comments, for example),
`*` marks the places that are already guarded before this patch,
`X` mars the places guarded in this patch, and
`^` marks places fixed in the next commit:

NO_WRAP
```
* ./src/lib/core/coio_file.c:509:	const char *tmpdir = getenv("TMPDIR");
X ./src/lib/core/errinj.c:75: const char *env_value = getenv(inj->name);
- ./src/proc_title.c:202: * that might try to hang onto a getenv() result.)
- ./src/proc_title.c:241:	* is mandatory to flush internal libc caches on getenv/setenv
X ./src/systemd.c:54: sd_unix_path = getenv("NOTIFY_SOCKET");
* ./src/box/module_cache.c:300: const char *tmpdir = getenv("TMPDIR");
X ./src/box/sql/os_unix.c:1441: azDirs[0] = getenv("SQL_TMPDIR");
X ./src/box/sql/os_unix.c:1446: azDirs[1] = getenv("TMPDIR");
* ./src/box/lua/console.c:394: const char *envvar = getenv("TT_CONSOLE_HIDE_SHOW_PROMPT");
^ ./src/box/lua/console.lua:771: local home_dir = os.getenv('HOME')
^ ./src/box/lua/load_cfg.lua:1007: local raw_value = os.getenv(env_var_name)
X ./src/lua/init.c:575: const char *path = getenv(envname);
X ./src/lua/init.c:592: const char *home = getenv("HOME");
* ./src/find_path.c:77: snprintf(buf, sizeof(buf) - 1, "%s", getenv("_"));
```
NO_WRAP

Part-of #7797

NO_DOC=security

- Use uint32_t for string length. Actually internally it cannot
take more that INT_MAX length, so uin32_t is enough. This change
makes the hash table a bit more compact.
- Rename mh_strnptr_find_inp -> mh_strnptr_find_str. I beleive it
makes it more understandable.

NO_DOC=refactoring
NO_CHANGELOG=refactoring
NO_TEST=refactoring

It never returns NULL anymore, because it uses xmalloc for memory
allocations.

It never returns NULL anymore, because it uses xmalloc for memory
allocations.

The modules subsystem hides some low-level operations under API.
In particular the modules subsystem is responsible for

 - modules lookup in Lua's "package.search" storage
 - modules caching to eliminate expensive load procedure
 - function symbol resolving

Because naming is intersecting with current module functions
sitting in box/func, lets rename the later to schema_module
prefix. We will use this prefix in next patches to point the
modules in box.schema.func are just a particular user of
the general modules engine.

Part-of #4642

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>