commands: Rename the command functions

Command functions are traditionally named like the command shortened
to four characters.  When this name collides with a keyword or library
function, we abbreviate more: brea(), rea().  A few are unabbreviated,
e.g.  execute().  A few have different names, e.g. explain(), not
list().

Commit 23726b379 (v4.3.0) suppressed a GCC warning about carg()
colliding with its built-in function.

Ron Koenderink reported Microsoft Visual Studio 2019 fails to link:
"_carg already defined in ucrtd.lib(ucrtbased.dll)".

Time to clean this up: rename the functions to c_FOO(), where FOO is
the unabbreviated name of the command.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>

src/lib/commands/fort.c

@@ -37,7 +37,7 @@
 #include "optlist.h"
 
 int
-fort(void)
+c_fortify(void)
 {
     int nunits;
     struct nstr_item ni;