Display modules (/.dm, /.dsrv, /.dmtt) each demand 262328 contiguous bytes per request: no __stklen

[mgrossmann]

Found while closing mvslovers/mvsmf#287. httpd.c:22 sets
unsigned __stklen = 64*1024; — but that only applies to the HTTPD main
program. The display modules are separate load modules with their own
@@crt0, and none of them sets it:

module	sources	__stklen	contiguous GETMAIN per request
HTTPD	httpstrt.c httpd.c httpprm.c	64 K	65584
HTTPDM (/.dm)	cgistart.c httpdm.c	—	262328
HTTPDSRV (/.dsrv)	cgistart.c httpdsrv.c	—	262328
HTTPDMTT (/.dmtt)	cgistart.c httpdmtt.c	—	262328
ABEND0C1	cgistart.c abend0c1.c	—	262328

httpd re-LINKs a CGI module on every request, so each /.dm or /.dsrv hit
demands a quarter megabyte of contiguous subpool 0 — for a module whose own
work is trivial. That is the demand that mvslovers/mvsmf#287 measured the
address space running out of: not storage, contiguity. Free holes of roughly
one stack each get fenced off, and a request that cannot find 262328
contiguous bytes fails U0801 / @@CRT1 - No storage for C stack before any
recovery exists.

These modules are also anvil-capable in the mechanism described in #195:
any request holding a 262 K stack for the duration is what other allocations
get planted beside.

Fix

One line per module, as httpd.c already does:

unsigned __stklen = 64 * 1024;

in httpdm.c, httpdsrv.c, httpdmtt.c and abend0c1.c.

Do not put it in cgistart.c. That looks like the tidy place — one line
covering every CGI module — but cgistart.c is autocalled out of
libhttpd.a by downstream CGI modules too, and at least one of them
(mvsMF, mvslovers/mvsmf#290) now defines __stklen itself. A definition in
the shared launcher would collide with theirs. It belongs in each module's
own translation unit.

Sizing

64 K is what httpd's own main task runs on, and mvsMF's handlers passed 544
curl tests on it. The display modules do far less than either. If any of them
needs more, /zosmf/test?fn=storage (mvsmf#289) reports the live figure out
of PPASTKLN, so the change is verifiable rather than assumed.

Refs mvslovers/mvsmf#287, mvslovers/mvsmf#290, #195.