Add string to mode_t parser

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);