Revert "basic/env-util: (mostly) follow POSIX for what variable names are allowed"

This reverts commit b45c068dd8.

I think the idea was generally sound, but didn't take into account the
limitations of show-environment and how it is used. People expect to be able to
eval systemctl show-environment output in bash, and no escaping syntax is
defined for environment *names* (we only do escaping for *values*). We could
skip such problematic variables in 'systemctl show-environment', and only allow
them to be inherited directly. But this would be confusing and ugly.

The original motivation for this change was that various import operations
would fail. a4ccce22d9 changed systemctl to filter
invalid variables in import-environment.
https://gitlab.gnome.org/GNOME/gnome-session/-/issues/71 does a similar change
in GNOME. So those problematic variables should not cause failures, but just
be silently ignored.

Finally, the environment block is becoming a dumping ground. In my gnome
session 'systemctl show-environment --user' includes stuff like PWD, FPATH
(from zsh), SHLVL=0 (no idea what that is). This is not directly related to
variable names (since all those are allowed under the stricter rules too), but
I think we should start pushing people away from running import-environment and
towards importing only select variables.

https://github.com/systemd/systemd/pull/17188#issuecomment-708676511

src/basic/env-util.c

@@ -21,21 +21,18 @@
         DIGITS LETTERS                          \
         "_"
 
-static bool printable_portable_character(char c) {
-        /* POSIX.1-2008 specifies almost all ASCII characters as "portable". (Only DEL is excluded, and
-         * additionally NUL and = are not allowed in variable names). We are stricter, and additionally
-         * reject BEL, BS, HT, CR, LF, VT, FF and SPACE, i.e. all whitespace. */
-
-        return c >= '!' && c <= '~';
-}
-
 static bool env_name_is_valid_n(const char *e, size_t n) {
+        const char *p;
+
         if (!e)
                 return false;
 
         if (n <= 0)
                 return false;
 
+        if (e[0] >= '0' && e[0] <= '9')
+                return false;
+
         /* POSIX says the overall size of the environment block cannot
          * be > ARG_MAX, an individual assignment hence cannot be
          * either. Discounting the equal sign and trailing NUL this
@@ -44,8 +41,8 @@ static bool env_name_is_valid_n(const char *e, size_t n) {
         if (n > (size_t) sysconf(_SC_ARG_MAX) - 2)
                 return false;
 
-        for (const char *p = e; p < e + n; p++)
-                if (!printable_portable_character(*p) || *p == '=')
+        for (p = e; p < e + n; p++)
+                if (!strchr(VALID_BASH_ENV_NAME_CHARS, *p))
                         return false;
 
         return true;