Long ago, sh used to have its own optimized and restricted string formatting
implementation, which the printf builtin had to bypass via asprintf() to a
temporary buffer. Since sh has used libc's string formatting implementation
for a long time, remove the workaround.
Add a check to keep printf %c '' working the same way (output nothing);
POSIX allows both outputting nothing and outputting a NUL byte.
Also, this change avoids silently discarding format directives for whose
output asprintf() cannot allocate memory.
0b70e952ae