Add string to mode_t parser

added new function string_to_mode(char *m_string, mode_t base) which
parses a given string and converts it to a mode_t.
If either + or - are part of m_string the permissions are either
added or removed from base.

Currently support for permision copy is missing (e.g. g=u),
but all other flags should work.

Format for m_string: either symbolic modes or octal representation.

symbolic modes:
[auog][[+-=][rwxst]*]

examples:
string_to_mode("u=rwx,g=rw,o=r", 0);
string_to_mode("a-x", 0777);
string_to_mode("0744", 0);
diff --git a/toys/mkfifo.c b/toys/mkfifo.c
index 17d9c2c..ab466fc 100644
--- a/toys/mkfifo.c
+++ b/toys/mkfifo.c
@@ -8,7 +8,7 @@
  *
  * TODO: Add -m
 
-USE_MKFIFO(NEWTOY(mkfifo, "<1", TOYFLAG_BIN))
+USE_MKFIFO(NEWTOY(mkfifo, "<1m:", TOYFLAG_BIN))
 
 config MKFIFO
 	bool "mkfifo"
@@ -22,16 +22,21 @@
 #include "toys.h"
 
 DEFINE_GLOBALS(
-	long mode;
+	char *m_string;
+	mode_t mode;
 )
 
 #define TT this.mkfifo
+#define FLAG_m (1)
 
 void mkfifo_main(void)
 {
 	char **s;
 
 	TT.mode = 0666;
+	if (toys.optflags & FLAG_m) {
+		TT.mode = string_to_mode(TT.m_string, 0);
+	}
 
 	for (s = toys.optargs; *s; s++) {
 		if (mknod(*s, S_IFIFO | TT.mode, 0) < 0) {