Clean up compiler warnings about print_level.

v2 - update based on Erez's review comments
v3 - reformat commit message

Gcc (and presumably other compilers) emits a warning about the
declarations of print_level in various main() function shadowing
the global declaration in print.c (made external in print.h).  This
warning is completely benign but makes it more difficult to recognize
warnings that might be consequential.

This patch renames local instances of print_level.  It also gets rid
of a redundant call to print_set_level() in ts2phc.c and relocates a
misplaced call to print_set_level() in phc_ctl.c.

Signed-off-by: Michael Galassi <michael@galassi.us>