progress.h: format and be consistent with progress.c naming

progress.h: format and be consistent with progress.c naming

Fix an inconsistency introduced in dc6a0757c4f (make struct progress
an opaque type, 2007-10-30) and rename the "progress" parameters to
stop_progress{,_msg}() to "p_progress". Now these match the
corresponding parameters in the *.c code.

While we're at it let's move the definition of the former below the
latter, a subsequent change will start defining stop_progress() in
terms of stop_progress_msg(). Let's also remove the excess whitespace
at the end of the file.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>