kldxref: Properly handle reading strings near the end of an ELF file

If a string is at or near the end of an input file and the amount of
remaining data in the file is smaller than the maximum string size,
the pread(2) system call would return a short read which is treated as
an error. Instead, add a new helper function for reading a string
which permits short reads so long as the data read from the file
contains a terminated string.

Reported by: jrtc27
Reviewed by: jrtc27
Sponsored by: University of Cambridge, Google, Inc.
Differential Revision: https://reviews.freebsd.org/D44419