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);

utils.c

@@ -501,6 +501,20 @@ char *fgetline(FILE *fp)
     return ret;
 }
 
+/*
+ * Read an entire file into a BinarySink.
+ */
+bool read_file_into(BinarySink *bs, FILE *fp)
+{
+    char buf[4096];
+    while (1) {
+        size_t retd = fread(buf, 1, sizeof(buf), fp);
+        if (retd == 0)
+            return !ferror(fp);
+        put_data(bs, buf, retd);
+    }
+}
+
 /*
  * Perl-style 'chomp', for a line we just read with fgetline. Unlike
  * Perl chomp, however, we're deliberately forgiving of strange