Reinstate revision 1.11:

Reinstate revision 1.11:
BDECFLAGS; ANSIfy; use EXEC instead of "exec " where appropriate; use
proper types (size_t, int); use proper variable names for certain things;
get rid of static 'cache' style stuff by moving and sanitizing the
original SHELL checking code to main(), this also makes it easier to
free() the string; rename file-scope system() to exec_shell(); use
snprintf() everywhere instead of sprintf(); actually remember to free()
other malloc()'d char pointers in main().

I left out the -s option in this revision along with getusershell()
checking because of objections made by Warner Losh <imp> and Garrett
Wollman <wollman>.  I agreed with their assertions that such code was
unnecessary in a program like this.

I dare people to make this coredump now.

Some suggestions:	nectar (snprintf() truncate checking)
Reviewed by:		markm, eivind, jedgar
Tortured by:		examples in apply(1), fuzz(1),
			and a lot of random ideas I came up with