unistd.h

[Konstantin8105]

Add functions:

• getopt
• optarg

Link with documentation:

• http://gel.sourceforge.net/examples/unistd_8h-source.php
• http://unix.superglobalmegacorp.com/Net2/newsrc/sys/unistd.h.html
• https://code.woboq.org/qt5/include/bits/getopt_core.h.html

Examples:

• https://www.gnu.org/software/libc/manual/html_node/Example-of-Getopt.html
• https://linuxprograms.wordpress.com/2012/06/22/c-getopt-example/
• http://nxmnpg.lemoda.net/3/getopt
• http://c.happycodings.com/gnu-linux/code11.html

[Konstantin8105]

Implementation in #588 is not correct, because cannot parse flag like -ab