Improve `doprnt' and its usage. (Bug#8545)

Improve `doprnt' and its usage.  (Bug#8545)

 src/doprnt.c (doprnt): Make sure `format' is never accessed beyond
 `format_end'.  Remove support for %l as a conversion specifier.
 Don't use xrealloc.  Improve diagnostics when the %l size modifier
 is used.  Update the commentary.
 src/eval.c (verror): Simplify calculation of size_t.
 src/coding.c (Ffind_operation_coding_system): Fix diagnostic error
 messages.