maint: don’t lseek memory objects

maint: don’t lseek memory objects

This affects behavior only on memory objects, which are not in Linux.
Formerly the code would use lseek on these objects,
but POSIX says the result of lseek is unspecified on them,
and in QNX lseek has undefined behavior on typed memory objects.
* src/head.c (elide_tail_bytes_file, elide_tail_lines_file):
Omit unnecessary uses of presume_input_pipe.
Improve some out-of-date comments.
(head): Do not assume a file is seekable merely because its
st_size is usable.  Instead, seek only on regular files.
* src/od.c (skip): Do not seek on memory objects.