Fix build warnings and possible bugs

Fix a few build warnings as well as some possible bugs in the package:

* common/scheduler/task.c: uses clamp() without util.h
* signer/src/daemon/engine.c: signal handlers should return void
* enforcer/src/daemon/engine.c: signal handlers should return void
* enforcer/src/daemon/engine.c: various invalid function pointer casts
* libhsm/src/lib/libhsm.c: hsm_ctx_new: memset size bug
* libhsm/src/lib/libhsm.c: hsm_get_error: possible unterminated string
10 jobs for master in 25 minutes and 53 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Check
passed #141208
updatepo

00:01:34

 
  Build
passed #141209
build

00:14:53

 
  Test
passed #141210
lintian

00:02:17

passed #141212
piuparts-install-mysql

00:03:12

passed #141211
piuparts-install-sqlite3

00:03:14

passed #141214
piuparts-upgrade-mysql

00:03:58

passed #141213
piuparts-upgrade-sqlite3

00:03:42

failed #141215
allowed to fail
reprotest

00:09:23

passed #141217
allowed to fail
shellcheck

00:01:44

passed #141216
allowed to fail
xmllint

00:01:40

 
Name Stage Failure
failed
reprotest Test
        if ( cleanup ); then :; else echo >&2 "cleanup failed with exit code $?"; fi;
fi
exit $__x
fi" failed with status 2
Uploading artifacts...
reprotest.log: found 1 matching files
reprotest/: found 15 matching files
Uploading artifacts to coordinator... ok id=141215 responseStatus=201 Created token=18bzxbZ5
ERROR: Job failed: exit code 1