fix TZ parsing logic for identifying POSIX-form strings

previously, the contents of the TZ variable were considered a
candidate for a file/path name only if they began with a colon or
contained a slash before any comma. the latter was very sloppy logic
to avoid treating any valid POSIX TZ string as a file name, but it
also triggered on values that are not valid POSIX TZ strings,
including 3-letter timezone names without any offset.

instead, only treat the TZ variable as POSIX form if it begins with a
nonzero standard time name followed by +, -, or a digit.

also, special case GMT and UTC to always be treated as POSIX form
(with implicit zero offset) so that a stray file by the same name
cannot break software that depends on setting TZ=GMT or TZ=UTC.

src/time/__tz.c

@@ -4,6 +4,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/mman.h>
+#include <ctype.h>
 #include "libc.h"
 #include "lock.h"
 #include "fork_impl.h"
@@ -154,10 +155,21 @@ static void do_tzset()
 	}
 	if (old_tz) memcpy(old_tz, s, i+1);
 
+	int posix_form = 0;
+	if (*s != ':') {
+		p = s;
+		char dummy_name[TZNAME_MAX+1];
+		getname(dummy_name, &p);
+		if (p!=s && (*p == '+' || *p == '-' || isdigit(*p)
+		             || !strcmp(dummy_name, "UTC")
+		             || !strcmp(dummy_name, "GMT")))
+			posix_form = 1;
+	}	
+
 	/* Non-suid can use an absolute tzfile pathname or a relative
 	 * pathame beginning with "."; in secure mode, only the
 	 * standard path will be searched. */
-	if (*s == ':' || ((p=strchr(s, '/')) && !memchr(s, ',', p-s))) {
+	if (!posix_form) {
 		if (*s == ':') s++;
 		if (*s == '/' || *s == '.') {
 			if (!libc.secure || !strcmp(s, "/etc/localtime"))