[Bug middle-end/94169] warn for modifying getenv() result

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94169

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
Other C standard functions that return a pointer to an unmodifiable object:

  localeconv(), setlocale(), strerror()

In addition, stdin, stdout, and stderr could be marked as pointing to read-only
(although uses by standard library macros would have to be exempted).

and POSIX:

  ctermid()
  dlerror()
  getgrent()
  gethostent()
  getnetbyaddr(), getnetbyname(), getnetent()
  getprotbyname(), getprotbynumber(), getprotent()
  getpwent(), getpwnam(), getpwuid(),
  getservbyname(), getservbyport(), getservent()
  getutxent(), getutxid(), getutxline()
  getgrid(), getgrnam(), getlogin()
  getsubopt() (object pointed to by second argument)
  nl_langinfo()
  ptsname()
  readdir()
  strsignal()
  ttyname()