ipc: Determine maximal size of a shared memory area at runtime.

ipc: Determine maximal size of a shared memory area at runtime.

During command dispatch, the afs process allocates shm areas for the
query result and passes the identifiers via the local socket to the
child process of para_server which is executing the command. If the
write to the (non-blocking) local socket fails, for example because
the call would block, afs closes the connection to the child process
immediately to avoid deadlocks.

Therefore the maximal output size of an (afs) command depends
linearly on the size of the shared memory areas, so it is desirable
to allocate areas as large as possible. Currently, we use the SHMMAX
if it is defined and fall back to the safe default value of 64K
otherwise. However, this default is much smaller than the typical limit
of 32M on Linux. Moreover, the maximal size of a shared memory area
(shmmax) can be set at any time on most if not all operating systems,
so runtime detection of shmmax is to be preferred.

Unfortunately the way to obtain shmmax varies between operating
systems. On Linux, the value is available as the contents of a file
in /proc while on {Free,Net}BSD and Darwin sysctlbyname() must be
called. Moreover, BSD and Darwin use different identifiers for the
sysctlbyname() call.

So any code that determines shmmax at runtime and works on all
of the above systems must necessarily be ugly. This patch tries to
concentrate all ugliness in the new shm_get_shmmax() function, so that
the changes outside of ipc.c consist only of replacements SHMMAX ->
shm_get_shmmax() and of the removal of the default SHMMAX setting.

The new function only determines shmmax once when it is called for
the first time.