hts_readlines() should check that the lines are textual

[jmarshall]

Some uses of samtools's read_file_list() are being recoded to use hts_readlines() instead.

This is mostly equivalent functionality, but read_file_list() checks that the lines are extant filenames and stops reading and fails if not. This is in response to samtools/samtools#21 (fixed by samtools/samtools@4b0058f), in which read_file_list() previously appeared to read forever when accidentally presented with a BAM file instead of a file list.

At the least, hts_readlines() needs to validate that it is indeed getting lines composed primarily of sensible printable textual characters. Perhaps also a mode (or an associated hts_readfilenames() function) that also checks that it is getting the names of extant files.