Find string data in read-only sections in addition to read-only segments

[cxiao]

There are some cases in which this plugin is not able to find any strings, due to not being able to find any read-only segments in the binary. There may also be cases where we need to look at sections in the binary, rather than segments.

For example, the plugin currently fails to find any strings inside the x86_64 slice of the Mach-O sample 9ca914b1cfa8c0ba021b9e00bda71f36cad132f27cf16bda6d937badee66c747.

This sample has all of its constant string data in the __const section of the binary. The __const section only contains read-only data; however, the segment that it is inside, located from 0x100000000 to 0x10039c000 is read-executable.

This is because this segment contains a number of sections, some of which are code and some of which are data:

Name	Start	End	Semantics
__text	0x100000fd0	0x1002b69d0	Read-only code
__stubs	0x1002b69d0	0x1002b6ed4	Read-only code
__gcc_except_tab	0x1002b6ed4	0x1002dcab8	Default
__const	0x1002dcac0	0x10038e4b0	Read-only data
__unwind_info	0x10038e4b0	0x10039beec	Default
__eh_frame	0x10039bef0	0x10039bfe8	Default

This binary actually also contains a read-only segment from 0x1003e8000 to 0x100598000 (the Mach-O __LINKEDIT segment, which contains metadata used by the dynamic linker dyld), but that segment contains no sections, and its contents are also not referenced from anywhere within any of the code inside the binary.