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/lib/lib.h b/lib/lib.h
index c219039..2f33236 100644
--- a/lib/lib.h
+++ b/lib/lib.h
@@ -132,3 +132,5 @@
 
 int sig_to_num(char *pidstr);
 char *num_to_sig(int sig);
+
+mode_t string_to_mode(char *mode_str, mode_t base);