printf is the wrong tool, use showargs

XXX probably not all right, discussion ongoing:
    http://thread.gmane.org/gmane.comp.standards.posix.austin.general/9972
XXX change all tests to not use printf again
Get rid of left over NULL elements in x_ftab as NELEM() is used instead.
From Martin Natano. Also get rid of related checks.
mark two ($@/$* expansion-related) testcases as expected-fail, with more to come ☹
fix buffer overflow found by zacts from IRC (bad length calculation)
now zsh and mksh are even… :|
rename the “tilde” function; apparently, some system headers define it
prototype for a static function used before it’s defined

XXX add protos for all static functions (just: hard to find them)
stop using ptrdiff_t
kill a Clang 3.2 scan-build workaround (completely; tested by valgrind and
gcc-snapshot’s sanitisers to never be a problem)
• Build.sh: fix NSIG detection for gcc-snapshot
• all: bump version to R50-current; add more comments; whitespace
• all: remove all mkssert(); we’ll do full re-runs of scan-build and,
  hopefully, Coverity Scan/Prevent
• check.t: fix a testcase (sed could exit false, but we don’t care)
• eval.c: fix tilde_ok data type (only unsigned may shl constantly)
• exec.c: fix shebang buf array accesses to always go via uint8_t *
no need for this explicit one, emacsfn.h already defines them
bit more on POSIX
fix shebang/magic decoding
oksh RCSID sync (nop because we are portable and thus still require <sys/file.h>)
int → bool
port this to GNU bash 1.12.1 from http://www.qemu-advent-calendar.org/#day-1
「if + comment(!) + label(!!) + one statement」 ← shouldn’t that get braces, style(9)?
fix here string parsing problem found by Stéphane Chazelas
OpenBSD RCSID merge, and logical follow one change:
handle unknown bases as ksh93 does: larger downgrade to 10
(although our max will stay 36, as ksh93 doesn’t have upper/lowecase)
and smaller downgrade for typeset -i, but not for arithmetics
oksh sync RCSIDs only:
• expr.c long done already
• mksh.1 does not apply to us (no character classes)
drop the pdksh binary package