security: check size boundaries for getenv() returns

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