Reinstate old-API functions needed by Bio-SamTools Perl module

bam_plbuf.o should be in libbam.a.  Reinstate several functions in
bam.h and remove residual bam_plp_set_mask() and bam_plbuf_set_mask()
declarations as htslib's underlying bam_plp_set_mask() disappeared in
samtools/htslib@cf12fbaa2e1958659c9c075d940eb16f698ccefd.

Reinstate sampileup(), which now does its own masking.  It still calls
bam_plbuf_push() for all reads, so that the resulting bam_plp_push()
has the opportunity to invoke overlap_remove() on masked-out reads.