New utility function to read a whole disk file.

I'm going to want this in a moment for Uppity, and it seems like the
sort of thing I should put straight into utils.c now, rather than
having to move it over later when I inevitably find another use for
it.

Rather than insisting on allocating a string buffer the way fgetline
does, it reads a whole file and transfers the result into an arbitrary
BinarySink, which works out the same if you use a strbuf at the call
site, but can do other things too if that turns out useful.

misc.h

@@ -97,6 +97,7 @@ static inline int toint(unsigned u)
 }
 
 char *fgetline(FILE *fp);
+bool read_file_into(BinarySink *bs, FILE *fp);
 char *chomp(char *str);
 bool strstartswith(const char *s, const char *t);
 bool strendswith(const char *s, const char *t);