Support host build for libdm and libfstab

The host builds for both libs are needed for libfs_avb host unit test.
Also replaces strlcat()/strlcpy() with snprintf() because the former
doesn't have a glibc version. Or switch char* to std::string*.

Bug: 112103720
Bug: 117960205
Test: m libdm ARCH=x86_64
Test: m libfstab ARCH=x86_64
Test: atest libdm_test
Test: atest fs_mgr_unit_test
Test: boot a device

Change-Id: Id9b92b5286b8ed9ab0d80f18ab5802dcfeb83dfa