Bump version to 3.9.x

Headed towards a 4.0.  The binary format will change slightly but
incompatibly (e.g. renumbering of intrinsics).  Having a new GC is
enough of a change to warrant a minor version bump, and it's been 5
years so why not go major.

* GUILE-VERSION (GUILE_EFFECTIVE_VERSION): Bump to 4.0.
(GUILE_MICRO_VERSION, GUILE_MINOR_VERSION): Bump to 3.9.0.
(LIBGUILE_INTERFACE_CURRENT, LIBGUILE_INTERFACE_AGE): Reset.

* meta/guile-4.0.pc.in:
* meta/guile-4.0-uninstalled.pc.in: Rename from guile-3.0.
* .gitignore: Ignore pkg-config files with 4.0 effective version.

* README: Update slightly.

* doc/ref/api-evaluation.texi (Load Paths):
* doc/ref/api-options.texi (Build Config):
* doc/ref/guile-invoke.texi (Environment Variables):
* doc/ref/history.texi (A Timeline of Selected Guile Releases):
* doc/ref/libguile-parallel.texi (Parallel Installations):
* doc/ref/srfi-modules.texi (SRFI-0): Update references to stable
version in paths.

* module/system/vm/assembler.scm (*bytecode-minor-version*):
* libguile/loader.h (SCM_OBJCODE_MAJOR_VERSION):
(SCM_OBJCODE_MINIMUM_MINOR_VERSION):
(SCM_OBJCODE_MINOR_VERSION): Use the same major objcode version for now
but bump the minor to 99.

* meta/guile.m4 (GUILE_PKG, GUILE_PROGS): Check for Guile 4.0.
* module/ice-9/boot-9.scm (%cond-expand-features): Add new cond-expand
features.

module/system/vm/assembler.scm

@@ -2323,7 +2323,7 @@ needed."
 
 ;; FIXME: Define these somewhere central, shared with C.
 (define *bytecode-major-version* #x0300)
-(define *bytecode-minor-version* 7)
+(define *bytecode-minor-version* 99)
 
 (define (link-dynamic-section asm text rw rw-init frame-maps)
   "Link the dynamic section for an ELF image with bytecode @var{text},