httpd: use register_commands()

Updates httpd_start() to use register_commands() for 'readform' and
'writeform' commands.  Adds server/httpd.h to export the new signatures
for this function (and httpd_stop), which allows removing the obsoleted
declarations inside openocd.c.

src/server/Makefile.am

@@ -14,6 +14,7 @@ libserver_la_SOURCES = server.c telnet_server.c gdb_server.c
 if HTTPD
 libserver_la_SOURCES += httpd.c
 endif
+noinst_HEADERS += httpd.h
 
 libserver_la_CFLAGS =
 if IS_MINGW