civetweb.c: change some local functions to static

Fix for cgcc warning:

src/civetweb.c:6189:22: warning: symbol 'mg_connect' was not declared.
  Should it be static?

src/civetweb.c:6743:6: warning: symbol 'get_system_name' was not declared.
 Should it be static?

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
main.c: use NULL for pointer

fix for cgcc warning:

src/main.c:238:48: warning: Using plain integer as NULL pointer
src/main.c:246:44: warning: Using plain integer as NULL pointer
src/main.c:418:17: warning: Using plain integer as NULL pointer

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
civetweb.c: use NULL for pointers

fix for cgcc warnings:

src/civetweb.c:5455:56: warning: Using plain integer as NULL pointer
src/civetweb.c:5696:28: warning: Using plain integer as NULL pointer
src/civetweb.c:5698:26: warning: Using plain integer as NULL pointer
src/civetweb.c:6456:34: warning: Using plain integer as NULL pointer
src/civetweb.c:6643:34: warning: Using plain integer as NULL pointer
src/civetweb.c:6811:29: warning: Using plain integer as NULL pointer

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>