Add netpbm image size obtaining and resizing

function mew-pbm-size
netpbm (PBM, PGM, PPM, PNM, PAM) formats

It is not necessary to convert PNM formats (including PBM, PGM, PPM) before resizing. 
So, their converters are not listed (nil is set).
And, this code also includes enabling it to resize when the format is PNM (pbm) even if the converter is nil.

However, it is necessary to convert PAM format.
Because, most emacsen don't support PAM format.
So, its converter pamtopnm is listed.
(Resizing tool pamscale supports PAM format but if its input is PAM format,
its output is PAM format, too. So, converter is needed.)