Many changes for ELF shared libraries on Linux and Android

Handling more than 2 PT_LOAD (and thus more than 1 executable PT_LOAD)
required extensive changes.  But the bonus is the infrastructure
to support a different (de-)compression algorithm for each PT_LOAD.

https://github.com/upx/upx/issues/341   i386 .so decompression
https://github.com/upx/upx/issues/609   armv7 .so on Android
https://github.com/upx/upx/issues/625   amd64 .so execution
https://github.com/upx/upx/issues/654   armv7 .exe
Not yet: .so on MIPS, PowerPC, PowerPC64

src/p_lx_interp.cpp

@@ -205,7 +205,6 @@ void PackLinuxElf32x86interp::unpack(OutputFile *fo)
     Elf32_Ehdr *const ehdr = (Elf32_Ehdr *) u.buf;
     Elf32_Phdr const *phdr = (Elf32_Phdr *) (u.buf + sizeof(*ehdr));
 
-    unsigned szb_info = sizeof(b_info);
     {
         fi->seek(0, SEEK_SET);
         fi->readx(u.buf, MAX_INTERP_HDR);