Suppress 'tainted string passed to vulnerable operation' false defects

Suppress 'tainted string passed to vulnerable operation' false defects

* include/private/gc_priv.h (TRUSTED_STRING): New tagging macro; add
comment.
* misc.c (GC_init): Process the result of GETENV("GC_LOG_FILE") by
TRUSTED_STRING.
* tools/if_mach.c (main): Process argv[3] by TRUSTED_STRING (before
passing the string to execvp).
* tools/if_not_there.c (main): Declare "fname" local variable;
process argv[1] and argv[2] by TRUSTED_STRING (before passing the
strings to fopen/opendir and execvp, respectively).