Copy several years of fixes from bfd/aoutx.h to bfd/pdp11.c.

* pdp11.c (some_aout_object_p): 4c1534c7 - Don't set EXEC_P for
files with relocs.
(aout_get_external_symbols): 6b8f0fd5 - Return if count is zero.
0301ce14 PR 22306 - Handle stringsize of zero, and error for any
other size that doesn't qcover the header word.
bf82069d PR 23056 - Allocate an extra byte at the end of the
string table, and zero it.
(translate_symbol_table): 0d329c0a PR 22887 - Print an error
message and set bfd_error on finding an invalid name string offset.
(add_to_stringtab): INLINE -> inline
(pdp11_aout_swap_reloc_in): 116acb2c PR 22887 - Correct r_index
bound check.
(squirt_out_relocs): e2996cc3 PR 20921 - Check for and report
any relocs that could not be recognised.
92744f05 PR 20929 - Check for relocs	without an associated symbol.
(find_nearest_line):  808346fc PR 23055 - Check that the symbol
name exists and is long enough, before attempting to see if it is
for a .o file.
c3864421 - Correct case for N_SO being the last symbol.
50455f1a PR 20891 - Handle the case where the main file name
and the directory name are both empty.
e82ab856 PR 20892 - Handle the case where function name is empty.
(aout_link_add_symbols): e517df3d PR 19629 - Check for out of
range string table offsets.
531336e3 PR 20909 - Fix off-by-one error in check for an
illegal string offset.
(aout_link_includes_newfunc): Add comment.
(pdp11_aout_link_input_section): ad756e3f - Return with an error
on unexpected relocation type rather than ASSERT.