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
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