This change adds internal APIs to allow png_warning messages to have parameters
 without requiring the host OS to implelment snprintf.  As a side effect the
 dependency of the RFC1132 code on stdio is removed and PNG_NO_WARNINGS does
 actually work now.

 the namespace.  Added png_get_current_row_number and
 png_get_current_pass_number for the
 benefit of the user transform callback.
 Added png_process_data_pause and png_process_data_skip for the benefit of
 progressive readers that need to stop data processing or want to optimize
 skipping of unread data (e.g. if the reader marks a chunk to be skipped.)

prototypes for most png_get_*() functions.

char_index, divisor, exp_b10, and gamma_val, respectively, and
changed exp() back to png_exp().

to avoid "shadow" warnings.  Revised png_fixed() in png.c to avoid compiler
warning about reaching the end without returning anything.