preventable SIGSEGV when bad AT_SYSINFO_EHDR

[John Reiser <jreiser@bitwagon.com>]

__dls3() and friends in musl/ldso/dynlink.c should check Elf headers more carefully.
I saw a SIGSEGV in decode_dyn() because vdso_base = ElfXX_auxv[{AT_SYSINFO_EHDR}].a_ptr
pointed to a region that was all zero, and thus vdso.dynv == 0.  The operating system
kernel is the only one who can perform a fork() or clone(), but other software can
perform execve().  In my case that other software had a bug.  However, the blame
for the SIGSEGV rests on __dls3() because it did not validate input data.  [This is
the stuff of exploits.]  Calling a_crash() is OK; but a preventable SIGSEGV must be
avoided, both directly and because it indicates a lack of secure implementation.

It is [mostly] reasonable that __dls3() should trust that a non-zero vdso_base points to
a region that is readable, is as big and as aligned as an ElfXX_Ehdr, and is const
(no other thread is writing it, neither is any other process via a shared memory mapping);
but after that ldso should check.

In particular, these should be checked:
   0 == memcmp(ELFMAG, &.e_ident[EI_MAG0], SELFMAG)
   .e_machine matches the executing ldso
   .e_ident[{EI_CLASS, EI_DATA}] match the executing ldso
   .e_phnum != 0
   .e_phentsize >= sizeof(ElfXX_Phdr);  and larger *IS ALLOWED*: derived classes, etc.
   .e_phnum * .e_phentsize is not too large  [loops that increment a pointer by .e_phentsize]
   .e_phoff >= sizeof(ElfXX_Ehdr);  overlap of Ehdr and Phdr is a logical error
   (.e_phoff + .e_phnum * .e_phentsize) < .st_size;  no access beyond EOF

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