argument names file_gamma, int_file_gamma, and srgb_intent consistently.

"png_ptr ptr" should be "png_structp png_ptr"

 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.)

put paramter descriptions in proper order, applied libpng indentation
style in code snippets.

if they need access to anything in zlib.h, and that a number of
macros such as png_memset() are no longer accessible by applications.

from png_charpp to png_bytepp in png_get_iCCP().  This change happened
in version 1.5.0beta36 but is not noted in the CHANGES.  Also, revised
the "synopsis" for png_get_iCCP() in libpng.3 accordingly.