implement PT_GNU_RELRO support
clean up internal dynamic linker functions enumerating phdrs

record phentsize in struct dso, so the phdrs can be easily
enumerated via it. simplify all functions enumerating phdrs
to require only struct dso. also merge find_map_range and
find_dso to kernel_mapped_dso function that does both tasks
during single phdr enumeration.
cosmetic improvements in dynamic linker cleanup

consistent use of braces in if/else structure, line length.
if dynamic linker's relro mprotect call fails, include reason in message